Combine butter, corn syrup and brown sugar in 2-quart batter bowl. Microwave on
high until mixture comes to a full boil. Stir. Microwave on high for 3 minutes
more. Stir in baking soda and vanilla. Pour mixture over popped popcorn in brown
paper bag. Shake. Microwave on high for 30 seconds. Shake. Tear open bag and
pour onto waxed paper.